Knowledge of the mechanisms cells use to transmit signals from the extracellular environment is crucial in understanding normal as well as aberrant cellular growth. Protein phosphorylation is intricately involved in transmitting signals initiated by many different types of extracellular stimuli. Studies of eukaryotic signaling pathways have focused on the contributions of serine, threonine and tyrosine phosphorylation of proteins. Data gathered from many laboratories indicate that phosphohistidine is a significant component of mammalian cellular protein. Histidine kinases and phosphatases have been implicated in mammalian signaling pathways in several in vitro studies. Recently, transient protein histidine phosphorylation has been demonstrated in vivo, in response to signaling pathways initiated by thrombin or collagen stimulation of human platelets. Appreciation of the molecular consequences of histidine phosphorylation for mammalian signaling requires identification of histidine kinases. The focus of this project is the biochemical purification of mammalian histidine kinases and molecular cloning of these enzymes. These studies have the potential to shed new light on the field of eukaryotic signal transduction.